1st error: you declare inFile in main, then you try to use it in a different method. variables declared in a method only exist within that method. If you want to share variables between methods you must declare them in the class, but outside any one method.
Fix this, and any other errors of the same kind, then see what's left.
ps: Please ensure that the code you post corresponds to the error messages, otherwize it's hard to use the line numbers in the error messages.

I declared inFile with most of variables now in class cause I want to share them for 3 methods. Appear that I have more errors. Especially still at
Scanner inFile = new Scanner(new FileReader("inputgpa.txt"));

You have lost your import for java.util.*, which is where Scanner is defined.
You still declare inFile in main, despite all that was said.
You now declare a String called inFile in your class. That's really confusing.
Finally
Your methods are static, which means they execute inthe context of the class itself, without any particular instance of the class.
You declare variables without the static keyword, so they belong to individual instances, so they cannot be used in static methods where there is no instance. That's why you get "non-static variable n cannot be referenced from a static context". Static methods need static variables.

You have lost your import for java.util.*, which is where Scanner is defined.
You still declare inFile in main, despite all that was said.
You now declare a String called inFile in your class. That's really confusing.
Finally
Your methods are static, which means they execute inthe context of the class itself, without any particular instance of the class.
You declare variables without the static keyword, so they belong to individual instances, so they cannot be used in static methods where there is no instance. That's why you get "non-static variable n cannot be referenced from a static context". Static methods need static variables.

What did you mean by "You declare variables without the static keyword,..."? May I have an example?

A field is either an instance field or a static field. An instance field belongs to a particular object, each time you make a new object of that class it has its own idNumber. Fields are instance fields by default.

If you prefix a field with the keyword "static":

static int idNumber;

you make it a static field. This means that there's only one of them, and all objects of that class have the same value for that field at any one time.

Methods are also instance or static methods. Instance methods are attached to one particular instance, which means they can refer to that instance's data. Static methods belong to the class as a whole, and therefore can refer only to static data and call static methods.

So when you declare your methods static so you can run them from your main method, you prohibit them from referring to the instance fields of the class.

This causes you a conundrum - how do you think you can run instance methods from the main method, if it's got to be static? Think about it a little and see if you can come up with a way to do that.

Ok!
I can't understand still, but...I just simplified it in hope that if you could walk me through from here would be easier for me.
I eliminated other methods down to just main, although I'd like to use it later for output of all students Grades.
Still have lots of errors

Ok!
I can't understand still, but...I just simplified it in hope that if you could walk me through from here would be easier for me.
I eliminated other methods down to just main, although I'd like to use it later for output of all students Grades.
Still have lots of errors

Alright, I cut off all the other methods now. compiled with no error, but when I run it give me some kind of problem where I have: idNumber = inFile.nextInt(); //student ID number. Appreciate any help. Thanks

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